A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The thin film transistor has been used in a display device typified by a liquid crystal television. A silicon-based semiconductor material is known as a material of a semiconductor thin film that can be used in a thin film transistor. Besides, an oxide semiconductor has attracted attention as another material.
As a material of the oxide semiconductor, zinc oxide or a substance containing zinc oxide is known. Thin film transistors each of which is formed using an amorphous oxide (an oxide semiconductor) having an electron carrier concentration of lower than 1018/cm3 are disclosed in Patent Documents 1 to 3.
A field-effect transistor in which an oxide semiconductor is used can be used in, for example, a display device. As examples of the display device, the following are given: a self-luminous display device for displaying an image by emitting light, a transmissive display device for displaying an image by selectively transmitting part of light from a backlight, and a reflective display device for displaying an image by reflecting external light.
The self-luminous display device and the transmissive display device are insusceptible to external light, emit bright light, and have excellent capability to display images.
The reflective display device has a feature in that power saving is easily achieved because it does not need to incorporate a light source. Needless to say, displayed images can be rewritten. Therefore, the reflective display device has attracted attention as electronic paper substituted for a print medium from societies aiming for saving of natural resources.
However, the visibility of the reflective display device is decreased in an environment where the amount of external light is small; thus, lighting is needed for using the reflective display device. The use of lighting damages the feature of the reflective display device of low power consumption. Hence, inventions, in each of which a substance that stores external light such as a light-storing fluorescent material or a light-storing material is used in a reflective display device for increasing the visibility in a dark place with power consumption suppressed, are disclosed in Patent Documents 4 and 5.
Reference
    [Patent Document 1] Japanese Published Patent Application No. 2006-165527    [Patent Document 2] Japanese Published Patent Application No. 2006-165528    [Patent Document 3] Japanese Published Patent Application No. 2006-165529    [Patent Document 4] Japanese Published Patent Application No. 2006-3924    [Patent Document 5] Japanese Published Patent Application No. 2008-116855